1. Field of the Invention
This invention relates to activated sludge systems and methods for the treatment of wastewater, and more particularly, to activated sludge systems and methods for the biological nutrient removal of phosphorus and nitrogen from wastewater using a stressed contact zone and a luxury contact zone.
2. Description of the Related Art
Wastewater treatment plants have long been subject to governmental regulation controlling levels of biochemical oxygen demand (BOD), total suspended solids (TSS), ammonia nitrogen (NH.sub.3) and dissolved oxygen (DO) in treated effluent. More recently, however, several of these plants have been faced with effluent permit limitations on nitrate nitrogen (NO.sub.3) and phosphorus (P). The primary reason for regulating nitrates and phosphorus is that these nutrients promote unwanted growth of algae and other aquatic plants. In order to control this eutrophication problem, governmental agencies will continue to enact increasingly stringent regulations further limiting the amount of such nutrients which can be discharged into receiving waters. Therefore, it is important to have a cost effective method for removing biological nutrients, such as nitrogen and phosphorus from wastewater.
One general method for removing nutrients from wastewater is biological treatment in an activated sludge system. One such method is the Bardenpho Activated Sludge Process. The Bardenpho Process requires five (5) different treatment zones: an anaerobic zone, a first anoxic zone, a first aerobic zone, a second anoxic zone, and a final aerobic zone. In the anaerobic zone, wastewater influent is combined with return activated sludge (RAS) to form a mixed liquor. The organisms in this mixed liquor consume BOD and release phosphorus into solution in the form of soluble inorganic phosphorus. When the mixed liquor moves into the first anoxic zone, where biological denitrification occurs, a mixed liquor recycle from the first aerobic zone brings nitrites and nitrates back into the first anoxic zone, where organisms consume the nitrate oxygen, producing nitrogen gas (N.sub.2) and nitrous oxide (N.sub.2 O). In the first aerobic zone, nitrifying organisms oxidize the ammonia to nitrite and nitrate, while the phosphorus-removing organisms take up phosphorus. The nitrogen and phosphorus removal process is continued further in the second anoxic and second aerobic treatment zones.
Another method for biological removal of phosphorus and nitrogen in an activated sludge system is the A/O process. The A/O process is essentially a simplified version of the Bardenpho Process, eliminating the second anoxic zone and second aerobic zone.
Concurrent biological nitrification and denitrification (CBND) provides yet another modified activated sludge method for biological nutrient removal. The CBND process calls for an anaerobic zone, an aerobic zone, an optional anoxic zone, and an anaerobic RAS holding tank. With the CBND system, RAS is held in the anaerobic holding tank until complete denitrification of the RAS has occurred. RAS is then combined with wastewater influent in the anaerobic contact zone to create a mixed liquor. Organisms release soluble inorganic phosphorus in this zone. The mixed liquor then moves into the aerobic zone, where various organisms achieve concurrent biological nitrification and denitrification, as well as phosphorus uptake. The optional anoxic zone allows for continued denitrification and phosphorus uptake. See Spector, U.S. Pat. No. 5,182,021.
More recently, a system called the POH Process has been developed for biological phosphorus removal. The POH Process achieves low effluent phosphorus levels through the use of three independently controlled processes. The first process is a mainstream activated sludge process that uses aeration and solids separation zones and return activated sludge to remove phosphorus. The second process is a side stream process for exposure of a portion of the RAS under anoxic conditions to remove nitrates followed by anaerobic conditions to allow organism selection and soluble phosphorus release. The third process involves a second side stream process having an anaerobic zone to satisfy the metabolic needs of the desired phosphorus removing organisms during the organism selection process. See WATER/Engineering & Management, August, 1994, page 15.
Although these various methods provide for the removal of phosphorus and nitrogen, they present practical limitations for many of the wastewater treatment plants across the country. These plants are not designed to provide the required elements, such as the number of treatment zones, sludge holding tanks, or mainstream and multiple side stream flows. Therefore, in order for these plants to meet nitrogen and phosphorus effluent limits using these processes, the respective municipalities would have to spend a great deal of money to modify the plants. In addition, all of the methods mentioned above are likely to have odor problems because of the use of anaerobic treatment zones. Because of the lack of DO in these anaerobic zones, many of the organisms decay, producing undesirable odors.
Therefore, it is desirable to have a biological nitrogen and phosphorus removal process which may be used in many existing activated sludge treatment facilities without significant expenditure to reconfigure the various plants. It would also be desirable to have a process which avoids potential odor problems due to anaerobic treatment zones.